It is known practice in the manufacture of internal combustion engines to cast the engine cylinder block using cores that comprise fine particles such as sand bonded by a resin. A core box is used to develop the semi-rigid cores that are used in a subsequent casting operation for forming the cylinder block.
The cores are destructible. The core material is removed from the cylinder block following the final casting step and may be recovered and reused in a subsequent casting procedure.
It is known practice to design the cores and the cylinder block casting mold package to create a coolant jacket cavity that surrounds only the region of the engine power cylinder that is subjected to high operating temperatures during operation of the engine rather than a coolant cavity that surrounds the entire cylinder wall. This makes possible rapid engine warm-up during engine operation.
The lower portion of the cylinder that is not surrounded by a coolant jacket is subject to external influences such as forces due to reinforcing ribs, bosses for attachment bolts and mounts for accessories which usually would extend directly to the cylinder bore wall. This creates a tendency to distort the cylinder bore walls which contributes to undesirable piston wear, reduced compression ratios and engine noise emanating from the lower regions of the engine cylinders. The lower region of the cylinders is not acoustically insulated by the relatively short water jacket formed by the cores during the casting operation.
An increase in the number of reinforcing ribs in prior art engines to reduce cylinder distortion and to increase stiffness of the engine cylinder block would result in an increased overall weight of the block and therefore would not be a feasible solution for the distortion problem.
The use of cores that define a short coolant jacket at the upper region of the cylinders and that provide a crankcase ventilation chamber near the lower region of the cylinders is disclosed in U.S. Pat. No. 5,253,615. The cores that are used in the casting operation in the manufacture of the cylinder blocks disclosed in the '615 patent make it possible to reduce distortion in the lower region of the cylinder wall by providing for a physical separation of the cylinder walls from bosses and other structural features that are required, for example, for attachment bolts and for mechanically supporting the engine while achieving structural rigidity of the cylinder block. Any distortion that is caused by structural bosses or ribbing or by the attachment of engine accessories to the engine block of the '615 patent is not directly transferred to the cylinder walls because of the crankcase ventilation chamber defined by the cylinder block casting cores positioned at the lower region of the cylinder walls.
The ventilation chamber for the engine disclosed in the '615 patent communicates with the engine crankcase so that engine crankcase gases are distributed in a circuitous path through the entire engine cylinder block.
The cylinder block ventilation chamber of the engine disclosed in the '615 patent is intended, therefore, as a dual purpose chamber. That is, it is intended to protect the lower region of the cylinders from outside influences, such as external forces caused by engine mounting bolts and structural ribbing, as well as to function as a positive crankcase ventilation passage. This dual purpose adds weight and complexity to the cylinder block.